A fossil-fuel power plant for generating electric energy produces a flue gas comprising carbon dioxide in particular by the combustion of a fossil fuel. To reduce the emission of carbon dioxide (CO2), a known secondary measure is to remove the carbon dioxide from the flue gas by means of a flue gas scrub. The absorption-desorption procedure is customary in particular. On a large industrial scale this involves the flue gas being contacted in an absorber with a scrubbing solution comprising an absorbent, scrubbing the carbon dioxide out of the flue gas into the scrubbing solution (CO2 capture process). The carbon dioxide is initially dissolved physically in the scrubbing solution and then chemically absorbed by the adsorbent. The scrubbing solution laden with carbon dioxide is subsequently sent into a desorber where, by raising the temperature, the carbon dioxide can be desorbed and, for example, sent into suitable storage. The absorbent is regenerated in the process, so it can be reintroduced into the absorber for renewed absorption.
Commonly used absorbents are based in particular on primary, secondary or tertiary amines or a mixture thereof, and display good selectivity and high capacity for the absorption of carbon dioxide. The chemical industry employs mainly the primary amine MEA (monoethanolamine) for its fast absorption kinetics. However, the energy efficiency of primary amines in an absorption-desorption process is poor, since the regeneration energy requirements are comparatively high. Since energy efficiency is not the primary concern in the chemical industry, the energy-related disadvantages could hitherto be very largely neglected. In power plants for power generation, by contrast, it is specifically the energy requirements of the CO2 capture process which are of considerable importance, since their magnitude has an appreciable influence on the overall efficiency of the power plant.
To avoid the energy-related disadvantage, therefore, the absorbents used for treating the flue gas of a power plant are sterically hindered amines (with regard to bicarbonate formation), secondary or tertiary amines, amino acid salts and/or potash solutions. These absorbents have distinctly reduced regeneration energies, which manifests itself in a lower efficiency drop for the power plant. In addition, secondary and tertiary amines have a higher loading capacity for carbon dioxide than primary amines.
However, the primary amine compounds do have the advantage of distinctly faster absorption kinetics. So, compared with secondary amines or amino acid salts, the absorber columns or reactors, in which the carbon dioxide is absorbed, can be designed smaller, which leads to lower capital costs. Sterically hindered, secondary or tertiary amines or amino acid salts have slower reaction kinetics because they form unstable carbamate products, if any at all.
The advantage of amino acid salts over heterocyclic amines or alkanolamines is in turn that amino acid salts have no noticeable vapor pressure and thus do not evaporate and can be dragged out into the environment during the separation process. Heterocyclic amines and alkanolamines are volatile and are dragged out by the cleaned flue gas as it passes into the environment, leading to adverse environmental impacts.